1. Field of the Invention
The present invention relates generally to a communication system utilizing a multiple access scheme, and more particularly to an apparatus and method for transmitting and receiving data in a multiple access communication system using a frequency hopping scheme.
2. Description of the Related Art
In the fourth generation (4G) communication system, which is the next generation communication system, research has been actively pursued to provide users with services having various qualities of service (QoS) and supporting a high transmission speeds. Particularly, in the current 4G communication system, research is being actively pursued to develop a new type of communication system capable of providing subscribers with high-speed services by ensuring mobility and QoS to broadband wireless systems, such as a wireless local area network (LAN) system and a wireless metropolitan area network (MAN) system.
Accordingly, many studies are being conducted on an Orthogonal Frequency Division Multiplexing (OFDM) scheme for high-speed data transmission over wired/wireless channels in the 4G communication system. The OFDM scheme, which transmits data using multiple carriers, is a special type of Multicarrier Modulation (MCM) scheme in which a serial symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal sub-carriers before being transmitted.
In order to provide high speed and high quality wireless multimedia services in the 4G communication system, wideband spectrum resources are required. However, the wideband spectrum resources are used, a fading influence on a wireless transmission line becomes serious due to the multipath propagation, and the influence due to frequency selective fading occurs even in a transmission band. Accordingly, for a high speed wireless multimedia service, the OFDM scheme, which is robust against frequency selective fading, has a relatively greater gain. Consequently, the OFDM scheme is being actively utilized in the 4G communication system.
A multiple access scheme based on the OFDM scheme as described above includes an orthogonal frequency division multiple access (OFDMA) scheme, which enables some of the sub-carriers to be allocated to a predetermined terminal. The OFDMA scheme does not require a spreading sequence for band spreading (spread spectrum). However, according to the OFDMA scheme, a sub-channel allocated to a predetermined terminal is fixedly maintained, such that the predetermined terminal may be influenced by continuous fading. Therefore, when the OFDMA scheme is used, it has a problem in that transmission efficiency is deteriorated.
Herein, the sub-channel represents a channel including at least one sub-carrier.
In order to solve such a problem, it is necessary to dynamically change a sub-channel allocated to a predetermined terminal depending on a fading characteristic of a wireless transmission line, thereby increasing a transmission efficiency based on a frequency diversity gain. Herein, dynamically changing sub-channels allocated to a predetermined terminal is called a ‘dynamic resource allocation’ scheme. A representative dynamic resource allocation scheme is a frequency hopping (FH) scheme.
When a channel of a system utilizing the OFDMA scheme is a quasi-static in which a channel state hardly changes, signals of a predetermined terminal, to which sub-carriers having a low channel gain are allocated, continuously suffer fading. In the following description, it is assumed that a multi-cell is based on the OFDMA scheme, uses a quasi-static channel, and has a frequency reuse factor of ‘1’.
First, because terminals located in a first cell of the multi-cell receive and use sub-channels different from each other, there is no interference between the terminals. However, terminals located in a cell, e.g., a second cell, adjacent to the first cell may use the same sub-channel as the terminals located in the first cell use, such that signals transmitted and received to and from the terminals located in the second cell may act as interference signals to the terminals located in the first cell. Therefore, when the FH scheme is connected with the OFDMA scheme to be used in a communication system, it is possible to prevent continuous fading and to prevent an interference signal from being received from an adjacent cell.
A scheme in which the FH scheme and the OFDMA scheme are connected with each other is called a frequency hopping orthogonal frequency division multiple access (FH-OFDMA) scheme.
According to the FH-OFDMA scheme, a sub-channel frequency allocated to each of terminals hops by using a predetermined FH code, thereby acquiring the effect obtained by the OFDMA, and also the effect obtained by the FH scheme.
Herein, the FH code may be a Latin-square code. The Latin-squire code is advantageous in that it distinguishes cells in a multi-cell environment and reduces inter-cell interference (ICI).
FIG. 1 is a block diagram schematically illustrating a transmission apparatus in a conventional FH-OFDMA communication system. Referring to FIG. 1, serial-to-parallel converters 102, 104, and 106 receive data streams generated from each terminal, and output a predetermined number of sequences (i.e., the length of a data stream) in parallel. A frequency hopper 108 receives signals output from the serial-to-parallel converters 102, 104, and 106, and dynamically changes sub-carriers of each sub-channel according to a predetermined frequency hopping pattern. Thereafter, an inverse fast Fourier transform (IFFT) unit 110 receives each of signals output from the frequency hopper 108, performs an inverse Fourier transform on the received signal from a frequency domain to a time domain, and outputs the transformed signal. The signal transformed to the time domain by the IFFT unit 110 is shifted to a signal of a radio frequency band through a normal OFDM transmission procedure, and is then transmitted.
The Latin-square code is the best-known FH code for the FH-OFDMA system. According to the Latin-square codes, each FH code set has a distinct slope, and a distinct FH code set is allocated to each cell. Therefore, a base station can differentiate cells by detecting the slopes of the FH codes.
A Latin-square code matrix includes a set of n codes, each of which has a length of ‘n’. Each of the n codes includes the numerals of ‘0’ to ‘n−1’ as its components. Every row and every column have all the numerals of ‘0’ to ‘n−1’ just once. When the ‘n’ is a prime number, an n×n Latin-square code matrix {a} may be expressed as shown in Equation (1).{a}ij=ai+j(mod n)  (1)
In Equation (1), ‘i’ and ‘j’ are parameters representing a row and a column of matrix {a}, respectively, and are used as a frequency index and a time index, respectively, when a frequency is hopped.
To generate the Latin-square code a first row is generated by arranging the numerals of ‘0’ to ‘n−1’ in sequence. A second row is generated by cyclically shifting the numerals of ‘0’ to ‘n−1’ by ‘a’ from the their positions in the first row. A third row is generated by cyclically shifting the numerals of ‘0’ to ‘n−1’ by ‘a’ from the their positions in the second row. Thereafter, the remaining rows can be generated by performing the above-mentioned process in sequence.
‘n−1’ number of Latin-square code sets that differ from each other can be generated by varying the value of ‘a’ from 1 to ‘n−1’. Herein, the ‘a’ represents a distinct slope of the Latin-square code sets. In the code matrix generated as described above, each column corresponds to an FH code. Therefore, each terminal is allocated with a distinct column of the code matrix, and performs frequency hopping to a frequency corresponding to a numeral included in the column at every period.
Although the FH-OFDMA scheme is used, the shortage of radio resources, a basic problem in a wireless communication, is not solved. That is, in order to accommodate more terminals and simultaneously perform high-speed and large quantity data transmission, it is necessary to research and develop a superior multiple access scheme. Accordingly, it is necessary to develop a new multiple access scheme, which can solve the resource shortage problem, maximize frequency diversity gain, and transmit a large quantity of data at high speed.